1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medicine images of a patient's body organs of interest. In particular, the present invention relates to a novel procedure and system for detecting the occurrence of valid scintillation events.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Emitted gamma photons are typically detected by placing a scintillator over the region of interest. Such scintillators are conventionally made of crystalline material such as NaI(TI), which interacts with absorbed gamma photons to produce flashes of visible light. The light photons emitted from the scintillator crystal are in turn detected by photosensor devices that are optically coupled to the scintillator crystal, such as photomultiplier tubes. The photosensor devices convert the received light photons into electrical pulses whose magnitude corresponds to the amount of light photons impinging on the photosensitive area of the photosensor device.
Not all gamma interactions in a scintillator crystal can be used to construct an image of the target object. Some of the interactions may be caused by gamma photons that were scattered or changed in direction of travel from their original trajectory. Thus, one conventional method that has been used to test the validity of a scintillation event is to compare the total energy of the scintillation event against an energy “window” or range of expected energies for valid (i.e., unscattered) events. In order to obtain the total energy of the event, light pulse detection voltage signals generated from each photosensor device as a result of a single gamma interaction must be accurately integrated from the start of each pulse, and then added together to form an energy signal associated with a particular event. Energy signals falling within the predetermined energy window are considered to correspond to valid events, while energy signals falling outside of the energy window are considered to correspond to scattered, or invalid, events, and the associated event is consequently not used in the construction of the radiation image, but is discarded. Without accurate detection of the start of an event, the total energy value may not be accurate, which would cause the signal to fall outside of the energy window and thereby undesirably discard a useful valid event.
Another instance of inaccurate information may arise when two gamma photons interact with the scintillation crystal within a time interval that is shorter than the time resolution of the system (in other words the amount of time required for a light event to decay sufficiently such that the system can process a subsequent light event as an independent event), such that light events from the two gamma interactions are said to “pile up,” or be superposed on each other. The signal resulting from a pulse pile-up would be meaningless, as it would not be possible to know whether the pulse resulted from two valid events, two invalid events, or one valid event and one invalid event.
Pulse pile-up may be manifested in different ways. Post-pulse pile-up describes a condition wherein a subsequent pulse occurs before processing of a pulse of interest is completed. Pre-pulse pile up describes a situation wherein a pulse of interest is overlapped by the trailing edge or tail of a preceding pulse. Post-pulse and pre-pulse pile-up conditions thus involve two pulses. A third type of pile-up situation is a combination of post-pulse and pre-pulse pile-up, wherein a pulse of interest is both overlapped by the tail of a preceding pulse, and interrupted by the occurrence of a subsequent pulse before processing can be completed. The combination pile-up thus involves three pulses.
To maximize efficiency and performance, it would be desirable to process all valid pulses (i.e., all pulses that correspond to unscattered gamma interactions with the detector, or “true scintillation events”). Where two valid pulses overlap, there would thus exist a post-pulse pile-up condition with respect to the first pulse of interest and a pre-pulse pile-up condition with respect to the subsequent pulse of interest. Further, where three valid pulses overlap, there would exist a post-pulse pile-up condition with respect to the first pulse of interest and a combination post/pre-pulse pile-up condition with respect to the second pulse of interest. In such circumstances the probability of the third pulse being a valid scintillation event would be very low and thus the third pulse desirably would be discarded and not processed.
Different solutions to the pulse pile-up problem are known in the prior art. One such solution involves the use of pile-up rejection circuitry, which either precludes the detector from processing any new pulses before processing has been completed on a prior pulse, or stops all processing when a pile-up condition has been identified. This technique addresses the problem of post-pulse pile-up. Such rejection circuitry, however, may undesirably increase the “deadtime” of the imaging system, during which valid gamma events are being received but are not able to be processed, thereby undesirably increasing the amount of time needed to complete an imaging procedure. It is also known to “fill-in” the missing tail of a pulse whose processing has been stopped because of the occurrence of a subsequent overlapping pulse, according to an approximation algorithm. Another known technique addresses the problem of pre-pulse pile-up, which uses an approximation of the preceding pulse tail to correct the subsequent pulse of interest by subtracting the preceding pulse tail.
Because piled-up events may be detected by multiple photomultiplier tubes, it is necessary to apply the unpiling algorithms to the pulse signals from each PMT as well as to the total pulse (which is a combination of the individual pulse signals from all the PMTs of the detector). Conventionally, execution of the unpiling algorithms has been performed by a data processor that serially processes the pulse signals from each PMT and the total pulse. Hence, the maximum sustained count rate of the detector is limited by the required computation time for each PMT pulse signal in the processor correction loop. This maximum sustained system count rate typically is lower than the maximum count rate of the detector.
Therefore, there exists a need in the art for a solution that improves the maximum sustained system count rate of a nuclear medicine imaging system while correcting for pulse pile-up conditions to the maximum count rate of the detector, so as to enable the imaging system to realize the maximum count rate capability of the detector.